A method and a system of the above-mentioned type are known from U.S. Pat. No. 5,025,860. In the known system, the natural gas is purified on a platform or a ship and is thereafter transferred in compressed and cooled form via a high-pressure line to a LNG tanker where the gas is converted to liquefied form by expansion. The liquefied gas is stored on the tanker at a pressure of approximately 1 bar, whereas non-liquefied residual gases are returned to the platform or ship via a return line. The high-pressure line and the return line, which extend through the sea between the platform/ship and the LNG tanker, at both ends are taken up from the sea so that the end portions of the lines extend up from the water surface through free air and at their ends are connected to respective treatment units on the platform/ship and the LNG tanker, respectively.
With this transfer arrangement the high-pressure line and the return line will be subjected to external influences of different kinds under the different operational conditions which may occur in practice. Difficult weather conditions with storms and high waves will place clear limitations on the system operation, as both security reasons and practical reasons will then render impossible disconnection of the lines from a LNG tanker having full loading tanks, and connection of the lines to another, empty LNG tanker. Under such weather conditions it will also present problems to keep the LNG tanker in position so that it does not turn or move and interferes with the lines. In addition, in arctic waters the lines may be subjected to collision with icebergs or ice floes floating on the water.
In offshore production of hydrocarbons (oil and gas) it is known to make use of production vessels which are based on the so-called STP technique (STP=Submerged Turret Production). In this technique there is used a submerged buoy of the type comprising a central bottom-anchored member communicating with the topical underground source through at least one flexible riser, and which is provided with a swivel unit for the transfer of fluid to a production installation on the vessel. On the central buoy member there is rotatably mounted an outer buoy member which is arranged for introduction and releasable securement in a submerged downwardly open receiving space at the bottom of the vessel, so that the vessel may turn about the anchored, central buoy member under the influence of wind, waves and water currents. For a further description of this technique reference may be made to e.g. Norwegian laying-open print No. 176 129.
Further, in offshore loading and unloading of hydrocarbons it is known to use a so-called STL buoy (STL=Submerged Turret Loading) which is based on the same principle as the STP buoy, but which has a simpler swivel means than the STP swivel which normally has several through-going passages or courses. For a further description of this buoy structure reference may e.g. be made to Norwegian laying-open print No. 175 419.
By means of the STL/STP technique there is achieved that one can carry out offshore loading/unloading as well as offshore production of hydrocarbons in practically all kinds of weather, as both connection and disconnection between ship and buoy can be carried out in a simple and quick manner, also under very difficult weather conditions with high waves. Further, the buoy can remain connected to the vessel in all kinds of weather, a quick disconnection being able to be carried out if a weather limitation should be exceeded.
Because of the substantial practical advantages involved in the STL/STP technique, it would be desirable to be able to make use of this technique also in connection with offshore production of liquefied natural gas. One could then construct a field installation for the production of LNG on a production vessel or a production platform, and transfer the liquefied gas to a LNG tanker via a transfer line and a STP buoy, as the LNG tanker then would be built for connection/disconnection of such a buoy. However, this is not feasible with the technique of today, since cryogenic transfer of LNG via a swivel, or also via conventional "loading arms", in practice is attended with hitherto unsolved problems in connection with freezing, clogging of passages etc. Such transfer is also attended with danger of unintentional spill of LNG on the sea, as this would be able to result in explosion-like evaporation ("rapid phase transition"), with a substantial destructive potential.
On this background it is an object of the invention to provide a method and a system for offshore production of LNG, wherein the above-mentioned weaknesses of the known system are avoided, and wherein one also avoids the mentioned problems attended with cryogenic medium transfer.
Another object of the invention is to provide a method and a system of the topical type which utilizes the STL/STP technique and the possibilities involved therein with respect to flexibility, safety and efficient utilization of the resources.
A further object of the invention is to provide a method and a system of the topical type which result in a relatively simple and cheap installation for conversion of natural gas to LNG.